Although thromboembolic disease is a leading cause of morbitity and mortality, the problem of thrombus detection is far from solved. Our approach concerns: 1. development of a simple isotope scanning technique, and, 2. enhancement of the diagnostic accuracy of angiography. Our first objective is the development of radioactive agents which injected parenterally will not carry infection risk and will depict both propagating and non-propagating thrombi. The interaction of thrombi with activators and other components of the fibrinolytic system or with their complexes is proposed to be used by employing these substances as carriers for radioactive iodine. Methods: preparation of starting materials in purified form, iodine labeling of streptokinase, urokinase, plasmin, plasminogen, Streptase R etc. directly or via small iodinated molecules, determination of enzymatic activity before and after labeling, determination of excretory pathways in animals, assessment of the uptake of materials into thrombi in vitro by binding studies and in vivo by scintiphotography of animals with experimental thrombi. The feasibility of the latter was demonstrated in animal experiments with labeled Streptase R. The second objective is to further develop and test an adhesive radiographic contrast material (ACM) ao that even minute thrombi will be detectable angiographically. We propose to modify the ACM previously developed by us for arteriography, for venous application. ACM adheres to the endothelium prolonging duration of the radiographic information and enhancing it by having different persistence on different surfaces. Methods: determination of optimal concentrations of meglumine diatrizoate and a special type of gelatin by physical measurements and by experimental angiography, determination of the method of sterilization, angiographic evaluation in animals with experimental small thrombi. In pilot experiments, minute thrombi obscured on conventional angiograms were detectable by ACM.